The present invention relates to a system for processing and transmitting pulse code modulation (hereinafter (PCM) signals, particularly audio signals which are recorded on a storage medium, such as a tape or disc, in PCM form and are played back therefrom.
Methods are known for recording audio or television signals as pulse codes modulated signals on a recording medium, for example a magnetic tape or a disc. Although it is possible in principle to subject data which are to be recorded or transmitted to a simple pulse code modulation and to transmit or record the coded data, this modulation method would be sufficient to meet the high demands for transmission quality only in an ideal transmission channel.
In reality, a magnetic tape, a disc or a transmission path are not ideal, but each distorts the signals to be processed and thereby adversely affects the fidelity of the resulting sound reproduction. The transmission path and the tape or disc will hereinafter be identified generally as the channel. Such a channel is a medium subject to interference, or noise, which generally distorts the signals in such a way that transmission is impossible without data security. There exist various possibilities to provide data security, which methods are sometimes used alone or together with others.
A customary method is carried out as follows: the data to be transmitted are serially divided into blocks each of the same number of data words. Each data block is supplemented by a so-called cyclic redundancy check (CRC) data word as well as synchronizing word. The CRC word is a cyclic code obtained by means of a polynomial representation. In this way it is possible to detect errors in a block terminated by a CRC data word. Such CRC data word circuits are commercially available as integrated circuits, e.g. a Fairchild 9401 circuit. To correct errors, a so-called parity word may be added to each block, so that together with the CRC data word it is possible to both detect and correct error containing data. In order to be able to effect a correction, data and parity words which belong together must be transmitted in different blocks because even a single bit error makes a complete block initially unusable.
Proposals have therefore been made to not transmit the total data on the channel in their original sequence but to delay individual data words alternatingly by a certain duration or by a multiple thereof and to transmit the delayed and the undelayed data words on the channel in succession. This is called interleaving.
Noise occurring in a recording device often produces so-called burst errors on successively transmitted data words or blocks. However, the interleaving process pulls apart, or breaks up, these burst errors so that when the data words are recombined in their final, correct, original sequence, the noise is, in effect, dispersed. The dispersed noise, or errors, can then be eliminated by parity checks or by means of an interpolation circuit. Such an interleaving method is described in a published report by the Audio Engineering Society, 64th Convention, Nov. 2-5, 1979, in New York City. Such interleaving makes it possible to essentially eliminate noise from the system output signals. However, some noise in the channel produces error events in the signal to be reproduced, which events may extend over a plurality of adjacent data words. In that case, genuine correction by parity comparison is not possible. However, at the same time the conditions for correction by means of interpolation are also particularly unfavorable since uninterfered-with supporting values are absent between the values to be replaced.